Such a process for the preparation of urea is, for instance, described in EP 0834501 A2.
This patent publication describes the use of an ejector in the line connecting a submerged condenser and a reactor to be able to place the reactor and other equipment in a process for the preparation of urea on ground level. Liquid ammonia is used as the motive agent for the ejector.
The problem with the process as described in the prior art is, however, that when there is a low availability of liquid ammonia there is no proper motive agent for the ejector available to convey the condensate solution from the submerged condenser to the reactor. Low availability of liquid ammonia occurs, for instance, during a start-up of the process and when the process can not run at its full capacity for other reasons. To let the process run under these circumstances in the process according to the prior art liquid ammonia needs to be dosed as a motive agent to the ejector to keep the process running. Consequently, when all the liquid ammonia is used as motive agent, no ammonia is available anymore to adjust the ammonia to carbon dioxide molar ratio (N/C ratio) elsewhere in the high-pressure synthesis section of the urea production process. Adjustment of the N/C ratio is needed to prevent that an excess of non-condensed gases from the submerged condenser causes process upsets in parts of the high-pressure synthesis section downstream of the submerged condenser. The process upsets result, for instance, in a too high N/C ratio in the urea solution that leaves the reactor and thus result in a low amount of ammonia that is converted to urea.
The invention is characterized in that a gas stream leaving the top of the submerged condenser is controlled by one or more controlling elements.
It has surprisingly been found that by controlling the gas stream leaving the top of the condenser by one or more controlling elements the pressure drop over the ejector decreases resulting in the need of less motive agent for the ejector. In this way the amount of liquid ammonia needed as motive agent for the ejector is minimized and thus liquid ammonia can also be used for controlling the N/C ratio elsewhere in the high-pressure synthesis section. In this way it is possible to run the reactor at a lower capacity than its full capacity at optimal synthesis conditions and thus having a good ammonia conversion to urea.